Recently, a communication environment like a base station, etc., of a wireless communication environment employs a system which uses an element like a superconducting filter and the like. In order to transmit an ultrahigh frequency signal having more than hundreds of Megahertz in an ultrahigh frequency circuit in such a system, a coaxial cable having low signal attenuation is widely used.
That is, since the coaxial cable has not only a stable impedance and low attenuation characteristic, but also a high frequency characteristic such as an excellent shielding effect with respect to noise and the like, the coaxial cable is suitable for a high frequency communication line that is used in a base station required for a communication through a mobile phone transmitting a high frequency signal in a microwave band.
First, a structure of the coaxial cable will be described. The coaxial cable includes an inner conductor having a thin and long metallic wire shape located at the inside of the center thereof, an insulator made of an insulating material for surrounding the outside of the inner conductor, an outer conductor having a hollow cylindrical shape made of a metallic material for surrounding the outside of the insulator and a sheath made of the insulating material for surrounding the outer conductor.
When a coaxial cable is erected or assembled, or when the coaxial cable is connected to a terminal device, etc., located at a predetermined position, it is necessary to perform a bending process on such a coaxial cable. In this case, since a metal tube like a copper pipe is used as the outer conductor, the bending process cannot be easily performed. Furthermore, it is required to employ a device for exclusive use, for example, a tool for exclusive use and so on for the bending process.
Accordingly, FIG. 1 shows a coaxial cable having the outer conductor 125 with a corrugated tube shape having corrugation crests and corrugation troughs formed thereon in order to cause the bending process to be easily performed.
                              signal          ⁢                                          ⁢          propagation          ⁢                                          ⁢          velocity                ∝                  1                                    relative              ⁢                                                          ⁢              dielectric              ⁢                                                          ⁢              constant                                                          (        1        )            Meanwhile, a signal propagation velocity of the coaxial cable and a relative dielectric constant of the cable have a relationship described in the expression (1) above.Here, the relative dielectric constant of the cable is minimized so as to increase the signal propagation velocity. Here, however, there is a limit to minimize the relative dielectric constant to more than a certain level only through a method of changing a ratio of composition of an insulator.
Additionally, as described above, in the coaxial cable with the outer conductor having corrugation crests and corrugation troughs, which have a square wave shape, if the insulator inside the outer conductor has an uneven shape, the relative dielectric constant of the cable becomes different in accordance with the sections of the coaxial cable. Therefore, a constant propagation velocity of a signal cannot be provided according to the change of the relative dielectric constant in the longitudinal direction of the coaxial cable.
Besides, in the coaxial cable with the outer conductor having corrugation crests and corrugation troughs, which have a square wave shape, as described above, a curved surface of the outer conductor causes the cable to be cut obliquely right and left instead of perpendicularly to the longitudinal direction of the cable. Moreover, an edge for cutting the cable slides on the curved surface of the outer conductor.